In order to support the frequency within a grid system, all power station operators have to maintain a specific power reserve which can be called up within a predetermined time, and there is a fundamental distinction in this case between primary control and secondary control. The additional power output is used to return a frequency to the nominal value again, when said frequency is below the nominal value because of the power demand that has not been covered (electricity demand in the grid system is greater than that available).
In this case, the requirements vary from one grid system to another. In accordance with the UCTE (Union for the coordination of transmission of electricity) rules, for example, 2% additional power output is required within 30 seconds (primary control, in general this is referred to as primary control in the range from 10 to 30 seconds, or in general as frequency control in the seconds range). This power reserve is in this case provided by various measures within the energy generator installations which are already connected to the grid system, and these measures lead to a deterioration in the performance of the energy generator installations, and to increased technical complexity, which results in costs.
Furthermore, rules also exist which are determined on the grid system for providing additional power in the range from more than 30 seconds to 30 minutes (secondary control, or in general frequency control in the minutes range) which can be provided, in addition to the installations which are already connected to the grid system by, for example, peak-load installations, which are kept at readiness, as well.
In the case of steam power stations (DKW), the reserves are maintained in the seconds range by the present-day prior art by, inter alia, throttling back the turbine valves, that is to say by storage of energy in the boiler, in such a way that, when required, additional power can be made available in the seconds range simply by further opening the turbine valves. At the same time, the burner power is also increased, in order to allow the suddenly increased power to also be provided in the long term, by cancelling the throttling back.
However, throttling back the valves has the side effect that this always has a negative influence on the installation efficiency and the absolute power during normal operation. Because increased burner power must be possible, this also leads to over design of the boiler with its auxiliary and ancillary installations, and to a poorer cost balance.
A further possible way, which is currently used, to provide the required power reserve at short notice (often in conjunction with the throttling back of the turbine valve) is so-called condensate build-up. In this case, the bleed steam to the low-pressure preheaters is reduced (possibly down to 0 kg/s). This steam mass flow is correspondingly available for electricity generation in the steam turbine set. The increased amount of condensate which is available in the condenser as a result is first of all temporarily stored in the so-called hotwell and, once the burner power of the boiler has been correspondingly increased, the hotwell level is turned to the normal level again.
At the same time, if required, the amount of condensate fed by the condensate pump is also reduced, in order to maintain the condensate temperature despite the reduced bleed steam. In order to allow this to be ensured overall, the fired boiler and its auxiliary and ancillary installations must be designed to be correspondingly larger (that is to say additional costs). The installation is accordingly operated in the partial load range during normal operation, thus leading to corresponding efficiency losses.